Saturday, 3 November 2012

Three types of twilight

Whilst the poles experience great extremes of
light and darkness for extended periods of time, those who overwinter there in
locations such as the Amundsen-Scott Polar station are probably more aware than
most of three different classifications of twilight. For periods before the sun
finally rises (which is around the 23rd September at the
Amundsen-Scott station) the darkness is lessened by prolonged periods of these
twilights. Just before sunrise, the brightest of the three is civil twilight.

Civil Twilight occurs when the sun is between
0 and 6 degrees below the horizon, immediately after the sun has set at dusk,
further classified as either civil dawn and civil dusk depending upon whether
the sun is setting or rising. It tends to encompass the limits where objects
can still be clearly distinguished without extra illumination. The brightest
stars are now visible.

One phase darker is Nautical Twilight when
the sun is 6-12 degrees below the horizon. Navigation at sea by the brightest of
stars and the viewable presence of a horizon is possible during the period of
nautical twilight. At it’s end (or beginning in the morning) large objects may
be discerned, but without detail.

When the sun’s centre lies 12-18 degrees
below the horizon, we are in the period of Astronomical Twilight. It is named
thus because, under clear conditions certain sky objects, such as nebulae and
galaxies, are still not properly visible. To the casual observer however,
astronomical twilight often appears as night proper, and this is obviously more
marked in towns and cities where light pollution affects the level of ambient
light in the sky where such features may never be visible anyway. Other
factors, such as the presence of a full moon, may also make discerning astronomical
twilight difficult.

The first stirrings of civil dawn from the summit of Ben Hope, with a bright moon in the South East.

The duration of twilight is affected by our
latitude, and at the poles, twilight can last for weeks. It’s no surprise
therefore that an awareness of these intermediate points of clarity and
obscurity is far greater for the inhabitants of those areas. Of course, the
vast majority of us experience twilight as a daily occurrence. It’s a time of
day I’ve always liked. I feel fortunate to live in a Northern region where our
twilights are prolonged. The twenty-minute change from day to night (or vice
versa) at the equator seems to me too abrupt for the sensualities of fading or
growing light to be savoured.

The beginnings of civil dawn on the summit of Ben Hope. The moon was obscured by cloud for much of the night, although the skies to the North were clear.

I enjoy in particular the way in which our
surroundings, away from the artificial lights of towns and cities, take on a
kind of granularity, the way the light seems to hover in front of and on top of
things as the light begins to fade at dusk. Greens intensify for a short time,
and a certain pleasure can be had from not reverting to other forms of
illumination, taking the capabilities of twilight to it’s limit.

2am. Jim escaping from the wind in the summit shelter cairn. He and the rocks are illuminated by the twilight rather than the moonlight.

Civil dawn, 2am, looking North East.The lights in the distance are those of the small village of Tongue.

When Jim and I climbed Ben Hope, Scotland’s most
northerly Munro (mountain over 3000ft) close to the summer solstice a few years
ago in the hope of experiencing a Scottish midnight sun by virtue of the
combination of latitude and altitude, the sun did in fact dip down below the
horizon. Setting off at 12.05am from our tent, just after the sun set, for the couple of hours it took us to climb the hill, we had no need
for extra illumination, being fairly sure of my feet and the terrain, as I had climbed
it once before in torrential rain and within thick cloud. For the entirety of the walk and
for a chilly hour sat at the summit cairn waiting for the sun to raise above the
horizon, we passed through elongated phases of what I now know were the periods of civil and
nautical twilight. We can’t have entered astronomical twilight as
I was always able to discern at least some detail in my surroundings. Jim found a scrap of paper in his pocket, a receipt, which became a marker and measure of his capacity to see in this twilight since he was able, at all times, to read the characters on that tiny slip of paper. In a
way, there is a place and time in the UK where nightfall never really comes.

Looking North.

We waited on the summit for a long while, but
with a low band of cloud on the horizon, a visible sunrise seemed never to come.
Growing increasingly cold, we decided to head back down the leeward side of the
hill (leeward, sun-wise) watching as colour slowly penetrated the ground below.
That sense of the granularity that I describe which veils objects infused the surface of the boulder field near the top and in our shelter for
the night at the cairn. Once civil dawn was nearing its end, the colour of the
short, wind stunted grasses was at it’s most intense and varied. In fact, in the full
light of day, a more bland descriptiveness takes over and objects are perhaps
too easily seen, and the lyricism of the indistinct is lost.

Note* most of these images were made within a relatively short time of one another at around 2am. I only made photographs during the hour spent on the summit, so the period of nautical twilight was not documented.

3 comments:

A poetic post. I had no idea that twilight was categorised thus. Like you, I have always enjoyed the transmutation of the day into evening, finding dusk, particularly on a fine winter's day a most lovely time.Is dusk the same as twilight? I, too, fell fortunate in living in the North, where dawn and dusk are extended events that one may take prolonged pleasure in.

Dusk is indeed the same as twilight, but it is a subcategorisation to differentiate it from dawn. Dawn is therefore the twilight period when the sun is rising with dusk being the twilight period when the sun is setting.